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Research Interests

Dr. Marisol Ripoll

Multiparticle collision dynamics (MPC)

The dynamics of complex fluids is often governed by the hydrodynamic behavior of the solvent. Due to a large separation of length and time scales between the atomic scale of the solvent molecules and the mesoscopic scale of the solute, direct simulation approaches with atomistic solvent are prohibitively costly in computer time. This has stimulated the development of several mesoscale simulation techniques in recent years.

MPC is a particle-based mesoscale simulation technique. The fluid is modeled by N point particles. Positions and velocities evolve in discrete increments of time. The algorithm consists of two steps. In the streaming step ythe particles move ballistically. In the collision step, the particles are sorted into collision boxes, and the velocities relative to the centre of mass motion perform a random rotation.

Polymers and polymer assemblies exhibit a unique behavior in flow which is related to their conformational degrees of freedom. Their flexibility leads to a simultaneous deformation of the polymer and the fluid flow field, which strongly affect each other. Technologically, these systems are interesting for a variety of applications, such as drag reduction by polymer additives, drug delivery systems, or as motor oil viscosity modifiers. Star polymers, where f linear polymers are anchored to a common center, are interesting because their properties can be tuned by varying the functionality f as well as the arm length.

Using a novel mesoscopic simulation technique, known as multi-particle collision dynamics (MPC), we study the effect of a shear flow applied to star polymers of different functionalities and arm lengths. We investigate the induced anisotropy, orientation and rotation of the star polymers as a function of the flow intensity, as well as the effect of the polymer motion on the surrounding fluid motion.

Colloids with an anisotropic elongated shape, rods, are one of the typical building blocks of a large family of systems known as liquid crystals. Liquid crystals have found a large number of technical applications due to their versatility and rich phase behavior.

Suspensions of rod-like colloids show in equilibrium an isotropic-nematic coexistence region. The location and width of this region depends on the strength of the attraction interaction between rods. By means of hydrodynamic simulations we study the behavior of this phase transition in shear flow.

The shear flow induces alignment in the initially isotropic phase, which narrows the phase coexistence region. A collective rotational motion is induced in the originally nematic phase. This effect contributes to maintain the phase separation. A combined simulation-experimental study shows the existence of a universal shape of the non-equilibrium phase diagram.